We investigated the mask-pattern and trimethylalminium (TMAl)-flow-rate dependence of the Al composition in AlGaN selective growth for novel InGaN multiple quantum well lasers having selectively grown ridge structures (RiS-type lasers). The Al composition decreased with increases of the local coverage ratio. This result was explained quantitatively by a simple model in which the Al is deposited as polycrystals on the mask without migration occurring and the Ga is concentrated into the window. Under a high TMAl flow-rate condition, this effect of lowering the Al composition was weakened by the high Ga consumption on the mask, and an desirable Al composition for GaN-based lasers was realized. The model can be used to control the composition of p-AlGaN layers for RiS-type lasers.

We investigated the effect of Ge and Si in Cl2 plasma on reactive ion etching (RIE) of GaN. The etched surfaces of GaN were smooth, and high etch rates of 0.63 m/min and 0.41 m m/min were obtained using a Ge plate and a Si plate, respectively. A rough surface was formed for the quartz plate without the Ge plate or Si plate. Optical emission spectroscopy revealed optical emissions related to GeClx+ions in Cl2 plasma with the Ge plate, to SiClx+ ions with the Si plate and to Cl+ ions without the Ge or the Si plate. It is considered that the GeClx+ions and SiClx+ ions for RIE of GaN plays an important role in obtaining a smooth etched surface of GaN, and that the high-energy Cl+ ion severely damages the GaN surface.

A method based on Schottky capacitance-voltage (C-V) simulation was developed to extract the polarization-induced charge density in modulation-doped AlxGa1-xN/GaN heterostructures. There are two characteristic slopes in the experimental and simulated C-V curves.The influences of the polarization-induced charge density, n-AlGaN doping level and the Schottky barrier height on the positions of the two slopes in the C-V curves are much different from each other. The polarization-induced charge density can be extracted accurately by fitting the experimental C-V curves. It is extracted to be 6.78 x 1012cm-2in modulation-doped Al0.22Ga0.78N/GaN heterostructures with the Al0.22Ga0.78N thickness of 30 nm or 45 nm. The charge density reducesto 1.30 x 1012cm-2in the heterostructure with the Al0.22Ga0.78N thickness of 75 nm. It is thought that the reduction of the polarization-induced charges at the heterointerface is due to the partial relaxation of the Al0.22Ga0.78N layer on GaN.

An AlGaN/GaN hetero field effect transistor (HFET) was operated at 20 A. Its on-state resistance was lower than that of a Si-based FET. GaN and related materials were grown by gas-source molecular beam epitaxy (GSMBE). Sapphire substrates were used for GaN growth. An undoped GaN/Al0.2Ga0.8N heterostructure wasgrown on the substrate. The sheet carrier density of two dimensional electron gas was 1x1013 cm2 and the mobility was 1200 cm2/Vs at room temperature. The breakdown field of undoped high resistive GaN layer was about 2.0 MV/cm. Al0.2Ga0.8N/GaN HFET for a large current operation was fabricated. The gate width was 20 cm and thegate length was 2000 nm. The electrode material of the source and the drain was Al/Ti/Au and the Schottky electrodes were Pt/Au. The distance between the source and the drain was 6000 nm. The maximum breakdown voltage of gate and source was 600 V. The on-state resistance of the HFET was about 2 mohm•cm2 at 100 V. The transconductance (gm) of this HFET was about 120 mS/mm. It was confirmed that the HFET with a gate width of20 cm could be operated at the condition of a large current operation.

Raman scattering (RS) spectroscopy is employed to characterize the effect of nitrogen on structural properties of GaNxP1-x alloy with nitrogen composition up to 3 %. Two-mode behavior of the alloy is clearly shown. The frequency of the GaP-like LO phonons is found to decrease with N composition as –1.13 cm-1x. This dependence is proposed to be largely due to the biaxial strain in the GaNP epilayers, as a result of lattice mismatch to the GaP substrate. The frequency of the GaN-like phonons is found to be more sensitive to nitrogen content, increasing with the rate of +2.6 cm-1x. The addition of nitrogen is also found to cause a dramatic quenching of the two-phonon Raman scattering and an appearance of the zone edge GaP-like vibrations. These effects are suggested to reflect local distortion in the GaNP lattice induced by nitrogen, as well as possible clustering of N atoms.

The vacuum evaporation of GaN thin films using GaN powder (5N) for GaN-based electroluminescence devices (ELDs) is reported. The crystal structures of the evaporated GaN layers were amorphous, which was confirmed from reflection high-energy electron diffraction (RHEED) patterns. Auger electron spectra revealed that the layers have excess Ga metal. Bluish-white light emission was observed from the GaN-based ELD under AC operation at RT. Although the emission intensity was weak, the electroluminescence spectra started from the band edge of h-GaN.

Highly conductive ZnO films were fabricated on p-GaN in a two-step process. First, zinc was thermally evaporated on p-GaN. Next, zinc film was oxidized in oxygen flow. To increase the conductivity of ZnO, nitrogen was introduced into zinc during its deposition. The above procedure proved successful in fabricating ZnO of the resistivity of ~1x10-3Ωcm and resulted in ohmic contacts of resistivity ~1x10-2Ωcm2 to low-doped p-GaN, and light transmittance of ~75% in the wavelength range of 400-700 nm.

We have successfully fabricated self-organized GaN nanotips by reactive ion etching using chlorine plasma, and have revealed the formation mechanism. Nanotips with a high density and a high aspect ratio have been formed after the etching. We deduce from X-ray photoelectron spectroscopy (XPS) analysis that the nanotip formation is attributed to nanometer-scale masks of SiO2 on GaN. The structures calculated by Monte Carlo simulation of our formation mechanism are very similar to the experimental nanotip structures.

We have investigated InxAl1-xN layers grown by plasma source molecular beam epitaxy (PSMBE) on c-plane sapphire (0001) substrates with thin (about 100 Å) buffer layers of AlN. The value of x varied from 0 to 1. For all these layers, high resolution X-ray diffraction scans (XRD) show no indication of face segregation, while the atomic force microscope (AFM) images of the structures demonstrate a large number of nanobumps. Our spectroscopic measurements (UV/VIS optical absorption and reflection spectroscopy) of these bumpy structures indicate additional peaks that can be only associated with additional energy levels. We ascribe these levels to electrons, which are confined and quantized near the nanobump tips by strong piezoelectric field caused mainly by biaxial strain of the AlN layer. The calculated energies of these quantized states are in a good agreement with the spectroscopic data.

Textured gallium nitride (GaN) films were grown on polished, polycrystalline and amorphous substrates in sub-atmospheric pressures, by direct nitradation of a thin molten gallium films using electron cyclotron resonance (ECR) microwave nitrogen plasma. C-plane texturing was achieved, independent of the substrate crystallinity. Single crystal quality GaN nanowires with diameters ranging from 40-50 nm were also synthesized using direct nitridation of thin gallium films with nitrogen plasma. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and Cross-sectional transmission electron microscopy (CS-TEM), high-resolution TEM (HRTEM) and Micro-Raman spectroscopy were used to characterize the synthesized gallium nitride films and GaN nanowires.